Process for preparing dinitrogen pentoxide

ABSTRACT

Process for producing dinitrogen pentoxide (N 2  O 5 ) consists of reacting a solution of dinitrogen tetroxide (N 2  O 4 ) in a volatile organic solvent, with a stream of an ozone-containing carrier gas. N 2  O 5  produced by reaction between the N 2  O 4  and ozone is transferred into the gas stream, and is thereafter condensed out of the gas stream by contact with further inert organic solvent. The latent heat of formation of N 2  O 5  is absorbed by the heat of vaporization of the solvent, so limiting increases in reaction temperature and suppressing the dissociation of the N 2  O 5 . In a preferred embodiment the reaction and absorption steps are performed in separate columns having organic solvent recalculating continuously through each with carrier gas flowing continuously from the reaction column to the absorption column.

This invention relates to the preparation of dinitrogen pentoxide (N₂O₅) by the reaction between dinitrogen pentoxide (N₂ O₄) and ozone.

N₂ O₅ is a powerful and selective nitrating agent, especially whendissolved in inert organic solvents. Hitherto, its use as a nitratingagent has been limited because of its relatively high cost and lowerthermal stability.

One process of N₂ O₅ preparation which is capable of producing a highpurity product with a relatively long shelf life is the reaction ofgaseous N₂ O₄ with ozone. Ozone is produced at a maximum concentrationof about 4 wt% in oxygen within a silent electrical discharge ozoniser,and is immediately reacted with a stoichiometric amount of N₂ O₄ vapourin an inert gas carrier to give N₂ O₅. The inert gas stream containingthe N₂ O₅ product is then brought into contact with a cold surfacemaintained at a low temperature of typically -70° C. to condense out theproduct as a solid, whilst at the same time minimizing losses.

There are several disadvantages with this process. These are:

1. Since the concentration of ozone in the carrier gas stream is verylow, the volume of carrier gas is high in comparison with the volume ofgaseous N₂ O₅ product. The potential for product losses in the carriergas is therefore high, which is one reason for employing a very lowrecovery temperature.

2. The reaction between N₂ O₄ and ozone is exothermic, which has adetrimental effect in the thermally unstable N₂ O₅ product.

3. The recovered product is a solid which is difficult to transport,store, and use.

4. A very low product recovery temperature of typically -70° C. must beused, which leads to high operating costs especially since the highvolume of carrier gas must also be cooled to the same temperature.

5. The product is condensed onto a cold surface whose thermal efficiencyfalls as the thickness of product builds up.

6. The maintenance of a stoichiometric ratio of N₂ O₄ and ozone isdifficult to achieve, especially when these reagents are present in suchlow concentrations. A slight excess of ozone is lost in the carrier gaswhich adds to operating costs, whereas a slight excess of N₂ O₄ leads toN₂ O₄ contamination in the condensed product.

7. Any water vapour present in the ozone/gas stream from the ozoniserproduces nitric acid contamination in the product by reaction with N₂O₅.

It is an object of the present invention in a first aspect to provide aprocess for generating N₂ O₅ by reaction between ozone and N₂ O₄ wherebythe aforementioned disadvantages are overcome or at least mitigated inpart.

Accordingly, a process for preparing N₂ O₅ comprises the steps of

(a) providing a solution of N₂ O₄ in a first body of a volatile inertorganic solvent,

(b) contacting the solution with a carrier gas containing ozone at atemperature sufficient to promote formation of N₂ O₅ in, and evaporationof solvent into, the carrier gas,

(c) contacting the N₂ O₅ -laden carrier gas with a second body of inertorganic solvent at a temperature below that of step (b) to condense theN₂ O₅ therein, and

(d) recovering the condensed N₂ O₅ within the solvent from step (c).

FIG. 1 of the Drawing represents an apparatus for carrying out theprocess according to the present invention.

The principal advantage of the gas/liquid interaction in step (b) isthat the heat generated during the exothermic reaction between ozone andN₂ O₄ in step (b) is both absorbed by intimate contact with liquidsolvent of relatively high specific heat capacity and counteracted bythe latent heat of vaporization of the solvent and N₂ O₄. In this way,large temperature increases are suppressed and thermal decomposition ofthe N₂ O₅ avoided. Although this means that the carrier gas becomescontaminated with solvent vapour, step (c) provides an effective meansof recovering this solvent by low temperature condensation into thesecond body of solvent whilst at the same time directly converting theN₂ O₅ into readily storable, transportable and usable form without anintermediate solids handling stage.

The chemical identity of the first and second bodies of solvent arepreferably the same to avoid the problem of one solvent contaminatingthe other. Preferably, this solvent comprises C₁ or C₂ chloroalkane,especially dichloromethane, or a chlorofluorocarbon, especially a C₁ orC₂ chlorofluorocarbon.

A further advantage of producing N₂ O₅ in step (b) is that any moistureintroduced within the carrier gas (which preferably comprises ozonisedoxygen or ozonised air) hydrates some of the N₂ O₅ and becomes trappedas relatively involatile nitric acid condensate within the liquidsolvent product of step (b). This obviates the need to provide a verydry source of carrier gas. If however the carrier gas is essentiallymoisture free then the produce of step (b) will not be contaminated withbniric acid, and since it too will contain some dissolved N₂ O₅ thissolvent produce may be blended with the N₂ O₅ -containing solvent ofstep (d) to form the principal product of the present method.

The carrier gas used in step (b) preferably contains a stoichiometricexcess of ozone of preferably up to 5%, more preferably from 0.5 to 2%,over that required to venerate the N₂ O₅. Any excess ozone present inthe carrier gas is carried over to and at least partly absorbed by thesecond body of insert solvent, the the result that it prolongs the shelflife of then₂ O₅ in the product solvent by reconverting its principalproduct of thermal decomposition (N₂ O₄) back into N₂ O₅.

Step (b) is preferably performed within the temperature range -20° C. to+30° c., more preferably -15° C. to +20° C., and most preferably at atemperature at which N₂ O₄ remains strongly colored in solution so thatits complete removable from solution can be detected by the absence ofcolor using, for example, colourimetric detection means. Strong solutioncolouration by N₂ O₄ is observed at temperatures at or above -15° C.,especially at temperatures at or above -10° C. At or above thesetemperatures N₂ O₅ in solution is essentially colorless and so itspresence in solution does not interfere with the colourimetric detectionof N₂ O₄. However, at temperatures above +20° C. the N₂ O₅ becomesincreasingly thermally unstable and so a maximum temperature of +10° C.is preferred in order to promote a reasonable rate of solvent andnitrogen oxide evaporation.

The carrier gas and first body of solvent are preferably contacted inco-current flow to promote continuous and efficient formation of N₂ O₅in the gas phase. Co-current flow ensures that the concentrations ofozone and N₂ O₄ in the carrier gas and solvent respectively are at amaximum at the beginning of co-current flow contact and at a minimum atthe end of that contact. The first body of solvent is preferablycontinuously recirculate in a closed loop to acilitare its replenishmentwith low concentrations of fresh N₂ O₄ and to facilitate the transfer ofN₂ O₅ into the gas phase. In order to provide a large surface area ofintimate contact between the first body of solvent and the carrier gasand to promote rapid vaporization of N₂ O₄ and solvent, step (b) isconveniently conducted in a first packed column.

Step (c) is preferably conducted at a temperature of -156° C. or less,preferably -20° C. or less, in order to ensure substantially completerecovery of N₂ O₅ from h carrier gas. At temperatures below -50° C. nosignificant improvement in N₂ O₅ and ozone removable from the carriergas occurs, but since the vapour pressure of the solvent steadilydeclines with decreasing temperature so that amount of solvent recoveredfrom the gas into the second body of inert solvent increases and this inturn obviates the need for a subsequent solvent recovery step. For thisreason, step (c) may be performed at temperatures typically as low as-70° C. though at the expense of increased cooling costs and increasedamounts of crystalline N₂ O₅ formed in solution. In order to promoteefficient recovery of N₂ O₅ from the carrier gas, the carrier gas andthe second body of inert organic solvent are preferably contacted incontinuous countercurrent flow, most preferably within a second packedcolumn. This has the added advantage of minimising the temperature of,and hence the solvent vapour concentration in, the spent carrier gas. Aswith the first body of solvent, the second body of solvent is preferablycontinuously recirculated in a closed loop to facilitate its reuse instep (c) and to promote the build-up of a high concentration of N₂ O₅therein.

The carrier gas is preferably provided as a single stream which contactsthe first and second bodies of solvent in sequence.

The concentration of ozone in the carrier gas is preferably at least 0.1wt% and will not normally exceed 4 wt%. In order to promote completereaction between the ozone and N₂ O₄, it is preferable that theconcentration of N₂ O₄ in the solvent is carefully controlled to ensurethat the vapour pressure of N₂ O₄ is approximately equal to the vapourpressure of ozone at the commencement of step (b). This requires thatthe average concentration of N₂ O₅ in the solvent is preferably between0.005 and 0.05 wt% and is more preferably between 0.01 and 0.02 wt%.close control over N₂ O₄ concentration can be maintained by continuouslyor intermittently adding N₂ O₄ at a known rate to the first body ofsolvent recalculating in a closed loop. The N₂ O₄ is preferably added asa concentrated solution of typically between 25 and 60 wt% in the sameorganic solvent as that of the first body so as to make up for solventlosses by evaporation into the carrier gas.

In a second aspect of the present invention, there is provided anapparatus for performing the process of the first aspect comprising areaction vessel having an ozone generation means a solvent supply meansconnected thereto, an absorber vessel having a cooling means connectedthereto and having a gas vent and a solvent product outlet extendingtherefrom, and a carrier gas transfer line connecting between thereaction and absorber vessels.

The apparatus preferably further includes a first solvent recirculationmeans connecting between a solvent inlet and a solvent outlet of thereaction vessel, and a second solvent recirculation means connectingbetween a solvent inlet and a solvent outlet of the absorber vessel.

The ozone generation and solvent supply means are preferably connectedin co-current flow to the reaction vessel whereas the solvent inlet andgas transfer lines to the absorber vessel and preferably connected incountercurrent flow. Each vessel preferably comprises a packed column.

Examples of the present invention will now be described with referenceto the following apparatus and process descriptions and to theaccompanying drawing

The apparatus illustrated in FIG. 1 comprises a feedstock reservoir 2, ametering pump 6 with a motor 8, a reactor column 10 containing packedsection 12, a reactor column recirculation pump 14, an absorber column16 containing a packed section 18, and an absorber column recirculationpump 20.

A reactor solvent recirculation line 22 extends between the bottom andtop of the reactor column 10 through the reactor column recirculationpump 14. Similarly, an absorber solvent recirculation line 24 extendsbetween the bottom and top of the absorber column 16 through theabsorber recirculation pump 20. Each of the reactor and absorberrecirculation lines has a product offtake line 26 and 28 respectivelyfitted with a valve 30 and 32 respectively. A feedstock transfer line 34connects between the reservoir 2 nd the reactor recirculation line 22through the metering pump 6. A solvent makeup line 36 connects with theabsorber recirculation line 24.

A gas inlet line 38 extends from an oxygen supply 40, through an ozonegenerator 42 nd into the top of the reactor column 10. At the top of theabsorber column 16 is a gas outlet line 44. The two columns areconnected below their respective packed sections by a gas transfer line46, and a liquid return line 48 having a valve 50. The absorber column16 is mounted slightly higher than the reactor column 10 to allow excessliquid collected in the bottom o the absorber column to overflow intothe reactor column through the return line 48 when the valve 50 is open.

Cooling coils 52, 54 and 56 are fitted, respectively, in the reservoir2, in the bottom of the reactor column 10 and in the bottom of theabsorber column 16 for controlling the temperature of the solvent atvarious parts of the apparatus.

A. BATCH PROCESS (SEMI-CONTINUOUS)

Process Description

1. With the equipment dry and purged with nitrogen to remove all tracesof moisture, a quantity of moisture-free inert organic solvent ischarged to the reactor column 10 and the absorber column 16.

2. The absorber column circulation pump 20 is switched on and thetemperature of the solvent in the absorber column 16 brought down tobelow -15° C. using the cooling coil 56, the actual temperature selectedbeing set by the partial pressures of the components in the gas streamto minimize process and solvent losses. The reactor column recirculationpump 14 is also switched on. Valves 30 and 32 are kept closed and valve50 is kept open.

3. The oxygen stream is then switched on from its source 40 to reducethe temperature of the solvent in the reaction column by evaporation,and the ozone generator 42 brought into operation and adjusted foroptimum conditions. The ozone-containing oxygen stream passes down thepacked section 12 of the reactor column 10 in co-current flow withreactor column solvent, through the gas transfer line 46, up the packedsection 18 of the absorber column 16 in countercurrent flow withabsorber solvent, and out through the gas outlet line 44.

4. Using the metering pump 6, the N₂ O₄ solution in the reservoir 2 iscontinuously metered into the circulating solvent of the reactor column10 and thereby flows down the packed column co-currently with theozonised gas stream. The relative flow rate of N₂ O₄ and ozone throughthe reactor column are selected to ensure that ozone is always presentin stoichiometric excess. Contact between solvent and gas streams causesa proportion of the ozone to dissolve into the liquid phase and reactexothermically with the N₂ O₄ to give N₂ O₅ in solution. At the sametime some of the N₂ O₄ and solvent vaporises, the vaporized N₂ O₄ thenimmediately reacting exothermically with the ozone in the as stream toproduce more N₂ O₅. The effect of metering the N₂ O₄ solvent intosolvent recalculating within a closed circuit is to keep the N₂ O₄concentration in the solvent entering the reaction column very low(typically between 0.01 and 0.02 wt%) to maintain a low N₂ O₄ partialpressure so that its concentration released into the gas stream byvaporization is approximately equal tot the low concentration of ozonepresent in the gas stream. The gas and liquid flow int h reactor column10 is co-current. This ensures that a the concentrations of the reactivecomponents (ozone and N₂ O₄) in these two streams are at a maximum attheir point of contact and at a minimum when the streams separate andpermits maximum contact time for the reaction to take place. In thisway, complete reaction between the ozone and N₂ O₄ is promoted, lossesare minimized, and the liberated heat of reaction between the N₂ O₄ andozone can be effectively and progressively counteracted by the coolingeffect produced any the evaporation of the solvent and the N₂ O₄ so asto suppress thermal decomposition of the N₂ O₅ once formed. Any watervapour present int the gas stream reacts with N₂ O₅ to form nitric acid,which being relatively involatile condenses out in the reactor columnsolvent.

5. By appropriate use of the cooling coils 52 and 54, the temperaturewithin the packed section 12 of the reactor column 10 is maintained at atemperature above that which N₂ O₄ and N₂ O₅ are effectively strippedout of solution (typically above -10° C.) and yet within a temperaturerange in which N₂ O₄ in solution is strongly colored brown. Thus, colorindicator 58 is used to adjust the speed of the pump 6 to ensure theflow of N₂ O₄ in solution to the reactor column 10 is lower than whichcauses a brown colouration in the solvent at the base of the column.This in turn means that the required excess of ozone is present in thegas stream since all the N₂ O₄ is being converted to N₂ O₅. the requiredconcentration of the N₂ O₄ solution in the reservoir is calculated withthe assistance of the level indicator 60 to ensure that the flow rate ofsolvent pumped through the pump 6 matches the rate of solventevaporation into theg as stream which is equivalent to a constantsolvent level indication in the reactor column 10.

6. In the packed section 18 o the absorber column 16, the gas stream,now laden with N₂ O₅ and saturated with solvent, contacts therecalculating low temperature absorber solvent in countercurrent flow.This causes the N₂ O₅, solvent, and excess ozone components within thegas stream to condense out into the solvent. Countercurrent flow ensuresthat the temperature of the gas, and thus the concentration of each ofthese components, is at a minimum as the gas leaves the column. As asafety measure, any excess solvent carried over from the reactor column10 to the absorber column 16 is allowed to return under gravity throughthe valve 50. This prevents accidental build-up to excess solvent withinthe absorber column 16.

7. When the required quantity of N₂ O₅ has been prepared, as judged by amass balance to give a final solution concentration at a desirable levelfor subsequent usage, the ozone generator 42 is turned off together withthe oxygen source 40 and the metering pump 6. The equipment is thenallowed to warm up (typically to between -20° C. and 0° C.) to ensurethat any crystals of N₂ O₅ formed in the absorber column 16 redissolvein the solvent. The hue 32 is then opened ad the N₂ O₅ solution in theabsorber column 16 drawn off through the offtake line 28.

8. The solution in the reactor column 6, if it is not contaminated withnitric acid, can be drawn of through the offtake line 26 with valve 30open and blended with the solution from the absorber column.

9. The equipment can then be recharged with fresh solvent and theprocess repeated.

EXAMPLE 1

Batch (Semi-Continuous)

N₂ O₄ (5kg) was dissolved in dichloromethane (5kg), to give a 50 wt%feedstock which was metered in to the absorber column at 0.38 kg perhour.

The reactor and absorber columns were charged with dichloromethane(41kg).

Solvent was recirculated through each column in a closed loop at a rate,typically 1200 kg per hour, calculated to be above the minimum wettingconditions and below the flooding conditions for the packed column used.

Oxygen was feed through the ozone generator to give 2 wt% ozone at anozone flowrate of 100 g/hr.

With the reactor column at 0° C. and absorber column at -50° C., thereaction was complete after approximately 26 hours.

The product output from the absorber column was a 12 wt% N₂ O₅ solution(40 kg) containing 0.2 wt% nitric acid.

The solution output from the reactor column was a 6 wt% N₂ O₅ solution(8 kg) containing 2 wt% nitric acid.

Typical vent losses were N₂ O₅ (0.1 kg) and dichloromethane (1.5 kg).

Typical system losses (ie remaining within system) were N₂ O₅ (0.3 kg)and dichloromethane (2 kg).

B. CONTINUOUS WITH VALVE 50 CLOSED

Process Description

1. Process steps 1 to 6 of the batch (semi-continuous) process areperformed as described above, except that product is taken offcontinuously through line 28 with valve 32 open, and fresh dry solventis continuously added through line 36 as required in order to maintainthe concentration of N₂ O₅ in the product as necessary.

2. The continuous process may be shut down intermittently if and when anunacceptably high concentration of nitric acid builds up in the solventwithin the reactor column, to be replaced by fresh acid-free solvent.The pump 6 is first switched off and reaction and evaporation within thereactor column allowed to continues of a shot wile to remove all traceof N₂ O₄ and N₂ O₅ from the solvent. Thereafter, the oxygen source 40,ozone generator 42 and recirculation pump 20 are switched off and thenitric acid-laden solvent in the reactor column 19 is drained outthrough the offtake line (with valve 30 open) for subsequent solventrecovery. Fresh dry solvent is then changed to the reactor column 19,and the continuous process restarted.

EXAMPLE 2

Continuous with Valve 50 closed

Feedstock consisting of 50 wt% N₂ O₄ in dichloromethane was metered intothe reaction column at 0.38 kg/hr.

The reactor and absorber columns were charged with dichloromethane (41kg) which was recirculated at 1200 kg/hr through each column.

Oxygen was fed through the ozone generator to give 2 wt% ozone at anozone flowrate of 100 g/hr.

Solvent within the reservoir, reactor column and absorber column wascontrolled at, respectively, 15° c., 0° C., and -25° C.

As soon as equilibrium conditions were obtained, product consisting of 6wt% N₂ O₅ and less than 0.5% nitric acid in dichloromethane was takencontinuously from the absorber column at a rate of 1.8 kg/hr.

The temperature of the absorber column was controlled at -25° C.necessitating additional solvent vapour recovery from the spent gasstream. The higher absorber temperature had the additional advantagethat it suppressed the formation of an N₂ O₅ slurry in the absorbersolvent which could otherwise block the pump 20 and line 24.

C. CONTINUOUS WITH VALVE 50 OPEN

Process Description

1. Process steps 1 to 6 of the batch (semi-continuous) process areperformed as described above, except that valve 28 remains shut, valve50 si kept open, and product is taken off continuously through line 26with valve 30 open.

2. the absorber column may be operated at a low temperature (typically-50° C.) at which an N₂ O₅ slurry will form, but since the slurry flowsinto the warmer absorber column it dissolves and so the product takenfrom the reactor column contains only dissolved N₂ O₅. Since any nitricacid formed in the process can only be removed with product, it ishighly desirable that the oxygen source supplies dry, essentiallymoisture-free oxygen to the ozone generator.

EXAMPLE 3 Continuous with valve 50 Open

Process conditions were identical to those of example 2, except that theflow of make-up solvent through line 36 was reduced to increase theconcentration of N₂ O₅ in the product (now taken from the reactioncolumn) from 6 wt% to 12 wt% now made possible by the higher temperatureof, hence higher N₂ O₅ solubility within, the product solvent.

We claim:
 1. Process for preparing N₂ O₅ comprising the steps of:(a)providing a solution of N₂ O₄ in a first body of a volatile inertorganic solvent, (b) contacting the solution with a carrier gascontaining ozone at a temperature sufficient to promote formation of N₂O₅ in, and evaporation of solvent into, the carrier gas, (c) contactingthe N₂ O₅ -laden carrier gas with a second body of inert organic solventat a temperature below that of step (b) to condense the N₂ O₅ therein,and (d) recovering the condensed N₂ O₅ within the solvent from step (c).2. Process according to claim 1 wherein the chemical identity of thefirst and second bodies of inert organic solvent are the same. 3.Process according to claim 1 wherein the solvent of the first and/orsecond bodies of solvent comprises a C₁ or C₂ chloroalkane or achlorofluorocarbon.
 4. Process according to claim 1 wherein the carriergas employed in step (b) contains a stoichiometric excess of ozone overthat required to react with the N₂ O₄.
 5. Process according of claim 4wherein the carrier gas contains a stoichiometric excess of ozone of upto 5%.
 6. A process according to claim 1 wherein step (b) is performedwithin the temperature range of -20° C. to +30° C.
 7. Process accordingto claim 1 wherein step (c) is performed within the temperature range of-15° C. to -70° C.
 8. Process according to claim 7 wherein step (c) isperformed within the temperature range -20° C. to -50° C.
 9. Processaccording to claim 1 wherein the solvent product of step (d) is blendedwith at least part of the solvent product of step (b).
 10. Processaccording to claim 1 wherein each of the first body of solvent andcarrier gas is provided as a separate stream.
 11. Process according toclaim 10 wherein the carrier gas and first body of solvent are contactedin step (b) in co-current flow.
 12. Process according to claim 10wherein the first body of solvent is recirculated.
 13. Process accordingof claim 10 wherein the second body of solvent is provided as a separatestream.
 14. Process according to claim 13 wherein the carrier gas andsecond body of solvent are contacted in step (c) in countercurrent flow.15. Process according to claim 13 wherein the second body of solvent isrecirculated.
 16. Process according to claim 1 wherein the concentrationof N₂ O₄ in the first body of solvent employed in step (b) is rom 0.005to 0.05 wt%.
 17. Process according to claim 6, wherein step (b) isperformed within the temperature range of -15° C. to +20° C.